In recent years, nonvolatile memories, which are semiconductor devices that keep stored data even if the power is turned off, are widely used. In a flash memory, which is a typical nonvolatile memory, each transistor constituting a memory cell has a floating gate or an insulating film called a charge storage layer. Data storage is performed by storing electrons in such charge storage layers. As a flash memory having a charge storage layer formed of an insulating film, there is a flash memory of a silicon-oxide-nitride-oxide-silicon (SONOS) structure in which a trap layer in an oxide-nitride-oxide (ONO) film stores electrons. As a flash memory of SONOS structure, U.S. Pat. No. 6,011,725 discloses a flash memory (related-art example) having a virtual ground memory cell that symmetrically drives a source and a drain that are replaced with each other.
FIG. 1 is a cross-sectional view of a flash memory according to the related-art example. With reference to FIG. 1, a semiconductor substrate 10 is provided with a bit line 12 used as both the source and the drain. On the semiconductor substrate 10 is provided an ONO film 20 made of a tunnel oxide film 14, a trap layer 16, and a top oxide film 18. On the ONO film 20 is a word line 22 that also serves as a gate.
As a method of writing data in the flash memory, there is a method of injecting electrons into the trap layer 16 using the hot electron effect or the Fowler-Nordheim (F-N) tunnel effect. Furthermore, as a method of erasing data, there is a method of removing electrons from the trap layer 16 using the hot hole effect or the F-N tunnel effect. Generally in the related-art example, the data writing method uses the method of injecting electrons into the trap layer 16 using the hot electron effect. According to this method, electrons can be injected into two charge storage areas 24 independently of each other, by interchanging the source and the drain between the bit line 12 (BL1) and the bit line 12 (BL2). Therefore, two bits can be stored in one transistor. In addition, the data erasing method generally uses the method of removing electrons from the trap layer 16 using the hot hole effect.
With high integration and miniaturization of memory cells, the length of a channel 26 is reduced resulting in an approach between two charge storage areas 24. Consequently, the electrons stored in the two charge storage areas 24 interfere with each other. As a result, it becomes difficult to distinguish the electrons stored in the areas from each other (that is, to read data distinctly).
In addition, with the method of data erasing using the hot hole effect, a punch-through phenomenon occurs when the length of the channel 26 is reduced. The punch-through phenomenon refers to a phenomenon in which the electric current becomes uncontrollable because the depletion layers of the bit line 12 (BL1) and the bit line 12 (BL2) connect with each other when a high electric field is applied between the bit lines 12 (between BL1 and BL2). Because of this phenomenon, the erasing efficiency of electrons significantly drops. If data erase is performed using the F-N tunnel effect, there occurs an apparent excessive erasing in which holes appear to accumulate in the middle of the trap layer 16 between the bit lines 12. This occurs because electrons are not accumulated in the middle of the trap layer 16 between the bit lines 12. With the method of accumulating electrons in the trap layer 16 using the hot electron effect, the excessive erasing continues once it occurs because electrons cannot be injected into the middle of the trap layer 16 between the bit lines 12. Therefore basically, the F-N tunnel effect cannot be practically applied for data erasing. As described above, the related-art example has a problem that it is difficult to highly densify and miniaturize memory cells.